Dispersed Calcium Oxide as a Reversible and Efficient CO2−Sorbent

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Dispersed Calcium Oxide as a Reversible and Efficient CO2-Sorbent at Intermediate Temperatures Philipp Gruene,† Anuta G. Belova,† Tuncel M. Yegulalp,† Robert J. Farrauto,†,‡ and Marco J. Castaldi*,† †

Earth and Environmental Engineering Department, Columbia University in the City of New York, 500 West 120th Street, New York, New York 10027, United States ‡ BASF Corporation, 25 Middlesex Turnpike, Iselin, New Jersey 08830, United States ABSTRACT: Dispersion of calcium oxide on high surface area γ-Al2O3 creates a stable and reversible CO2-sorbent that overcomes the problems associated with bulk CaO, such as limited long-term stability, slow uptake kinetics, and energy-intensive regeneration. This sorbent is a candidate for the sorption-enhanced hydrogen production via steam reforming and/or water-gas shift reactions. CO2 uptake tests were performed in a 15% CO2/N2 atmosphere to evaluate the efficacy at typical hydrocarbon reformer gas partial pressure. CO2 uptake kinetics and capacities are investigated in TGA studies, while the long-term stability is shown in multicycle experiments. The dispersed CaO is an active sorbent at low temperatures and binds CO2 at 300 °C up to 1.7 times as efficiently as compared to bulk CaO powder. Furthermore, the sorbent can be regenerated in a CO2-free atmosphere at intermediate temperatures between 300 and 650 °C. Multicycle CO2 uptake and release has been tested for 84 cycles at a constant temperature of 650 °C and shows the superior long-term stability of dispersed CaO as compared to bulk CaO. The attempt to increase the uptake capacity from 0.16 to 0.22 mmol CO2 per gram of sorbent occurred with a commensurate loss in BET area from 115 to 41 m2, leading to a decline in overall uptake efficiency from 15% to 6%. Infrared spectroscopy is used to characterize the CO2-sorbent binding interaction on a molecular level and to distinguish between CO2 adsorbing on the bare support, on bulk CaO, and on dispersed CaO/Al2O3.

’ INTRODUCTION Increasing energy demands coupled with a heightened awareness of human influence on the global climate system, particularly the production of greenhouse gases, has resulted in incentives to develop alternative sources of energy and chemicals and to address energy security in an environmentally benign manner. One direction that is evolving to satisfy the demand is to move toward a hydrogen economy. However, for the foreseeable future, hydrogen will be generated through the increased usage of fossil fuel reforming. This realization has raised some questions about the responsible use of fossil fuels, and consequently what is emerging is the critical need for a carbon dioxide management strategy and technology to forestall projected global warming and its impacts.1-5 Hydrogen can be produced from fossil fuels or biorenewable sources through steam reforming (SR) processes, for example, fossil or biogas methane: CH4 þ H2 O T CO þ 3H2

CaO þ CO2 T CaCO3

ΔR H ¼ þ 206:2 kJ=mol ð1Þ

To enhance the hydrogen yield and remove CO from the product gas stream, the water-gas shift (WGS) reaction occurs in a subsequent reactor: CO þ H2 O T CO2 þ H2

promote complete reactant conversion, it is desirable to remove one of the products from the effluent. A significant amount of investigation has been done on hydrogen removal in situ and ex situ. Recently, the attention has been focused on the possible removal of the CO2 product, as this can also facilitate carbon sequestration efforts. The conventional way this has been designed is CO2-removal by using either monoethanolamine or pressure-swing adsorption on zeolites integrated in a staged reactor system. Almost 10 years ago, it was shown experimentally that hydrogen production can be facilitated drastically by sorptionenhanced reactions (SER).8,9 This yields the possibility of a CO2-sorbent added to the SR and WGS catalyst in a single reactor, thus shifting the thermodynamic equilibrium entirely toward the product side. It is known that CaO is a promising sorbent, which reacts stoichiometrically with CO2:

ΔR H ¼ - 41:2 kJ=mol

ð2Þ

In both processes, catalysis is of fundamental importance, as it allows SR and WGS reactions to proceed at reasonably low temperatures and high rates.6,7 However, in both reactions, hydrogen production is ultimately limited by the thermodynamic equilibrium. This is particularly acute for the WGS reaction. To enable a more efficient reaction process and r 2011 American Chemical Society

ΔR H ¼ - 177:8 kJ=mol

ð3Þ

Calcium minerals are also among the most abundant metal oxides in nature and are commonly found in the form of carbonates, for example, limestone and dolomite. Heating these materials to very high temperatures (>900 °C) leads to liberation of CO2 and to the formation of calcium oxide. At lower temperature, the oxide can react again stoichiometrically with CO2, acting as a very cheap, robust, and high-temperature sorbent with very high sorption capacities.4 Unfortunately, bulk CaO as a CO2-sorbent Received: June 8, 2010 Accepted: January 27, 2011 Published: March 04, 2011 4042

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Industrial & Engineering Chemistry Research suffers from three major drawbacks. (i) Once the first layer of the oxide has formed the carbonate, the kinetics of the carbonation reaction, despite being highly exothermic, become slow because the reaction is limited by the diffusion of CO2 through the CaCO3-skin formed.10,11 Attempts to improve the uptake kinetics of CaO-based sorbents by innovative synthesis methods12 or precursors13 have been reported. (ii) Regeneration of the sorbent, that is, desorption of CO2, is very energy-intensive (typically over 800 °C for CaCO3 decomposition), so excessive sintering and mechanical failure of the oxide occurs. This leads to a well-documented loss in adsorption activity after a few sorption/regeneration cycles.14 The energy required to continuously convert CaCO3 to CaO typically results in a complex process that must be operated near theoretical efficiencies. However, if the CaO sorbent can uptake CO2 without forming bulk-like CaCO3, then repeated high temperature treatments, to release the CO2 and restore the CaO, can be avoided. This is a potentially large energy savings because the CaCO3 will only require a single, initial high temperature treatment, to form CaO once, but subsequent uptake and release of CO2 can be done at lower temperatures. Therefore, the longer the sorbent material can be utilized, the smaller the energy penalty will be of forming the initial CaO from CaCO3. (iii) Calcium oxide, to be an efficient sorbent, must be used in a form amenable to high surface area exposure such as a powder. While it is possible to operate with powders in fluidized beds, the pressure drop associated with them is very large. In addition, working with fine powders can be problematic due to entrainment in the process flow and attrition of the material. It has been realized that adding dopants15 and binders to enhance the structural stability of CaO-based sorbents can improve their regeneration properties. For example, it was found that calcined dolomite, calcium oxide containing MgO, becomes a more efficient sorbent than calcined limestone, pure CaO, after only four adsorption/regeneration cycles.16,17 Following up on this idea, mixed CaO-based sorbents have been synthesized by wet chemistry, either by mixing CaO powder with an Al3þ solution,18-21 by wet impregnation of an alumina22 or silica support23 with a Ca2þ solution, by coprecipitation of Ca2þ and Al3þ or Mg2þ salts,24 by wet mixing of Ca and Mg salts,25 or by dry mixing of Ca salts and MgO.26 For such sorbent materials to be economically acceptable, the minimum uptake capacity has been estimated to be above 0.5 mol/kg.3 Herein, we report on the synthesis of dispersed CaO/Al2O3, prepared by incipient wetness impregnation of γ-Al2O3 and subsequent calcination. Such sorbents behave fundamentally different from bulk CaO, circumventing the three drawbacks listed above, adsorbing CO2 efficiently at low temperatures as shown by thermogravimetric analysis. Further, the sorbent can be regenerated efficiently at much milder conditions, for example, in a CO2-free atmosphere at 650 °C. This leads to an improved long-term stability of the sorbent and makes it an ideal candidate for the sorption-enhanced production of hydrogen via steam reforming and water-gas shift reactions. An additional advantage of the highly dispersed sorbent material for future applications is its amenability to high adhesion immobilization on optimal structures such as ceramic monoliths and metal-wire meshes,27 which eliminates solids handling and the associated problems. The system is envisioned to be a swing reactor with parallel monoliths, each with a layer of sorbent admixed with a suitable water-gas shift catalyst. Once saturated with CO2, this catalyst-sorbent combination would be taken off line for steam

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regeneration, while a parallel reactor continues to enhance hydrogen production. Furthermore, the enhanced structural stability of thin washcoats on ceramic or metal monoliths would prevent the structural deterioration currently experienced with CaO particulates and pellets.17 Metal supports can allow localized electrical heating, which further reduces the energy needed for regeneration. The different binding interaction of CO2 on the dispersed CaO/Al2O3, as compared to bulk CaO, is investigated by in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and shows evidence for different pathways of adsorption for CO2 on bulk CaO and on the dispersed CaO/Al2O3.

’ EXPERIMENTAL SECTION The dispersed CaO/Al2O3 was prepared by incipient wetness impregnation of a mesoporous γ-Al2O3 (SBA-150, Sasol) carrier with a saturated Ca(NO3)2 solution. After 3 h of drying at 150 °C, the dispersed CaO/Al2O3 is obtained by calcination in air at 700 °C for 4 h. Higher loadings of CaO on γ-Al2O3 were achieved by repeated impregnation/calcination steps. The surface areas of the prepared sorbents were measured by nitrogen adsorption/desorption isotherms at -196 °C and standard multipoint BET analysis methods using a physisorption/chemisorption analyzer (Quantachrome Nova 2200 Surface Area Analyzer). Prior to adsorption, the samples were dried and degassed by heating in a vacuum to 350 °C for 3 h. The pore volumes were estimated from the desorption branch of the isotherms, applying the BJH method in the software package Quantachrome NovaWin. CO2 uptake experiments were done using a Netzsch STA 409 PC Luxx TGA instrument. 60 mg of the sorbent sample was heated to 700 °C in a dry N2 atmosphere until a constant weight was achieved. After being cooled to the desired temperature to start the investigation, a 15% CO2 (UHP grade, TechAir) in N2 (UHP grade, TechAir) gas mixture was introduced into the instrument using rotameters (Netzsch Gas Control; total flows ∼100 mL/min). The CO2 uptake is expressed as an adsorption efficiency θ, in which the number of adsorbed CO2 molecules is related to the number of CaO sites: θ¼ ¼

nðCO2, ads Þ  100% nðCaOÞ ðm - m0 Þ=44 g=mol  100% xload  mo =56 g=mol

ð4Þ

where m0 and m are the masses of the sorbent before and after CO2 uptake, respectively, and xload is the CaO-loading expressed in weight percent of the sorbent. DRIFT spectroscopy was performed using a Bio-Rad Excalibur spectrometer equipped with an MCT detector and a temperature controllable Spectra-Tech diffuse reflectance high temperature chamber with ZnSe windows. Samples were heated to 450 °C in a dry N2 atmosphere for 1.5 h. After being cooled to the desired temperature, a continuous flow of 15% CO2 in N2 gas mixture was introduced into the instrument (total flows ∼100 mL/min), and spectra were taken for 60 min. After that, the flow was switched back to pure nitrogen, and spectra were taken for another 60 min. A spectrum, recorded just before CO2 was introduced into the cell, was used for background subtraction. 4043

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Figure 1. Adsorption efficiency θ of sorbents at 300 °C. The left panel (a) shows the performance of bulk CaO powder, while the right panel (b) shows the adsorption/desorption results for dispersed CaO/Al2O3.

’ RESULTS AND DISCUSSION 1. CO2 Uptake Kinetics and Capacity. The CO2-uptake kinetics of bulk CaO powder and the CaO/Al2O3 sorbent have been investigated via thermogravimetric analysis. Figure 1 shows the results as adsorption efficiency versus test time for a gas switch testing protocol. Upon introduction of 15% CO2 in N2, which corresponds to a typical CO2 concentration during high temperature water-gas shift from a hydrocarbon autothermal reformer, CaO powder readily adsorbs carbon dioxide (Figure 1a). Two distinct kinetic regimes are observed for CO2 sorption, a very rapid uptake in the first 2 min followed by a slower adsorption, which is probably limited by CO2 diffusion through the newly formed calcium carbonate crust.10,11 At 300 °C after 30 min, an adsorption efficiency θ of ∼9% is reached; that is, 9% of all CaO sites adsorb a CO2 molecule. Once the gas atmosphere is switched to pure N2 at 300 °C, no mass loss is observed, indicating that the CaO 3 CO2 adsorbate is stable. For liberation of CO2, high temperatures typically greater than 800 °C are required. Figure 1b shows the adsorption/desorption behavior of a 5.7 wt % CaO/Al2O3 sorbent. Two major differences between the dispersed (b) and the bulk CaO (a) are observed. The first is that the adsorption kinetics of CO2 remain fast for higher CO2 loadings in the case of dispersed CaO likely because most of the CaO sites are located on the sorbent surface and are thus directly accessible. Therefore, diffusion limitations are less significant. Moreover, the overall uptake efficiency is considerably higher with 15.4% of all CaO sites adsorbing a CO2 molecule. The second major difference can be seen once the CO2 is switched off after 30 min and replaced with N2. The sorbent is partially regenerated at only 300 °C. In the case of the dispersed CaO, CO2 is obviously much more loosely bound and starts to desorb at temperatures as low as 300 °C; yet, while loosely bound it has higher uptake. The CO2 desorption is slower than its adsorption, and even if the desorption time is increased, roughly 50% of the adsorbed CO2 is bound irreversibly at 300 °C. This points toward the presence of multiple CO2-adsorbates that differ in binding energy, similar to what has been proposed for promoted hydrotalcites.28,29 The difference in the adsorption and desorption rate decreases with increasing temperature, and if the adsorption/desorption cycle is run at 650 °C (consistent with reaction conditions of catalytic steam reforming) at 30 min intervals, over 90% of the adsorbed CO2 is released.30 By extending the desorption period (∼60 min), the sorbent can be regenerated entirely, which means that if multiple CO2 adsorbates are formed, they all desorb at

intermediate temperatures of 650 °C. Under these conditions, the adsorption capacity of CaO/Al2O3 decreases to a value of 9%, while the CO2 uptake capacity of bulk CaO powder increases. This behavior points to major differences in the CO2-sorbent binding mechanism. While CO2 uptake by bulk CaO is an activated process with a considerable activation barrier leading to a highly stable carbonate, the binding of CO2 to the dispersed CaO/ Al2O3 does not require much activation energy and leads to a rather weakly bound complex. The adsorption/desorption characteristics of dispersed CaO/Al2O3 resemble sorbents like promoted hydrotalcites,31-36 rather then bulk CaO. It must be noted that for a very similar CaO/Al2O3 sorbent, synthesized upon wet impregnation, a CO2-adsorption efficiency of over 90% at 900 °C has been reported, in marked contrast to the present results.22 CO2 uptake by CaO species as shown in Figure 1 has been achieved in a dry reaction atmosphere. Obviously, during SR and WGS reactions, steam will be added to the reaction mixture. Preliminary results have shown that the dispersed CaO/Al2O3 is stable toward steam at moderate temperatures.37 Furthermore, the adsorption capacity is increased by up to a factor of 3 when 20% steam is added during CO2 adsorption,37 similar to what has been noted for bulk CaO, attributed to the catalytic formation of Ca(OH)2.38 The regeneration of the sorbent with pure steam would be desirable when, in an industrial application, a pure, ready-to-sequestrate CO2 stream is preferred, which could be generated upon simple condensation of water. If the sorbent is primarily used for sorption enhanced hydrogen production (i.e., equilibrium adjustment reactions), this point is less important. Obviously, the maximum amount of CO2 that can be adsorbed per mass of total sorbent (CaO/Al2O3) is lower when using dispersed CaO/Al2O3 as compared to pure CaO, which contain a smaller amount of the active compound. High CaO loadings are therefore desirable. A series containing 5.7, 11.7, 16.0, 20.3, and 23.8 wt % of CaO has thus been synthesized upon repeated impregnation steps, each one followed by calcination at 700 °C. Table 1 shows the surface area, the pore volume, and the uptake expressed in moles of adsorbed CO2 per kilogram of sorbent as well as in efficiency per CaO site. The γ-Al2O3 carrier is a very poor CO2-sorbent at 300 °C (Table 1), and the uptake reported is almost entirely due to the presence of CaO. While for loadings from 5.7 to 20.3 wt %, the CO2 uptake increases with CaO loading from 0.16 to 0.22 mol/kg, the efficiency of the uptake per CaO site decreases continuously from 15.4% to 6.0%. The decrease in efficiency goes along with a decreasing surface area and pore volume, attributed to narrowing and clogging of the pores due to 4044

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Table 1. Properties of CaO/Al2O3 Sorbents Prepared by Incipient Wetness Impregnation

a

loading [wt %]

surface areaa [m2/g]

pore volumeb [cm3/g]

CO2 uptakec [n(CO2)/kg(sorbent)]d

CO2 uptake efficiencyc [%]

0

133

0.45

0.04

5.7

115

0.40

0.16

15.4

11.7

88

0.33

0.19

9.3

16.0

62

0.25

0.19

6.6

20.3

41

0.17

0.22

6.0

23.8

39

0.17

0.15

3.4

BET multipoint analysis. b BJH method from desorption branch. c At 300 °C after 30 min in 15% CO2/N2 atmosphere. d n(CO2) = moles of CO2.

Figure 2. (a) Long-term stability of the CO2-adsorption efficiency during multiple temperature cycles between 200 and 850 °C. The experimental data are compared to bulk CaO powder and two models by Abanades14 and Wang.48 (b) Long-term stability of the CO2-adsorption efficiency during multiple CO2-partial pressure cycles between 0 and 150 mbar at atmospheric total pressure. The adsorption efficiencies are normalized to the first cycle.

deposited CaO. The lowering of the surface area is not due to sintering of γ-Al2O3 at the 700 °C calcination, as a series of sorbents with similar loadings but calcined at only 500 °C shows an almost identical surface-area dependence on the loading.39 Further increase of the loading does not lead to an increase but instead to a lowered CO2 sorption capacity. In the present series, this happens at a loading of 23.8 wt %. Interestingly, the decreased capacity is not accompanied by a more drastic loss in surface area or pore volume. It should be noted that also for the sorbent with the highest CO2 capacity, the uptake per sorbent mass remains below 0.5 mol/kg, which has been estimated to be an economically acceptable sorption working capacity.3 The adsorption capacity of 0.22 mol CO2/kg sorbent after 30 min at a p(CO2) = 150 mbar and 300 °C can be compared to equilibrium CO2-uptake measurements under similar conditions for K2CO3-promoted hydrotalcites (0.266 mol/kg at p(CO2) = 0.2 atm and 400 °C)35 and Na2O-promoted alumina (0.273 mol/kg at p(CO2) = 0.2 atm and 350 °C).40 Future optimization of the uptake capacity is therefore desirable and might be improved by modifications of the carrier, and the selection of adsorbent or the synthesis method employed. 2. Regeneration and Long-Term Stability. Besides a reasonable uptake capacity, an industrially relevant sorbent for CO2 capture must retain its activity over a large number of adsorption/desorption cycles. Bulk CaO, like calcined limestone or dolomite, but also synthesized samples, are known to lose their activity quickly.10,17,41-46 This has been shown to be due to sintering during high-temperature regeneration and a loss of micropore volume.14,43,47,48 The presence of binders and the high dispersion of CaO/Al2O3 effectively inhibit sintering and can lead to improved long-term stability.18,19,22,24 Figure 2a compares the efficiencies of dispersed and bulk CaO sorbents (the CO2 uptake is normalized to the uptake in the first cycle) in capturing CO2 in a thermogravimetric analyzer over

20 adsorption/desorption cycles. In each cycle, the sample was heated from 200 to 850 °C (20 K/min) while being exposed to a 50% CO2 in N2 atmosphere. Once at 850 °C, the gases were switched to 100% N2 for the sorbent regeneration, and the sorbent was cooled again to 200 °C (20 K/min). Under these conditions, the CaO powder sample quickly deactivates as reported previously.10,17,41-46 Its adsorption capacity drops to below 50% after 20 sorption/regeneration cycles. It has been shown that almost all sources of bulk CaO follow a very similar deactivation behavior. Thus, empirical models have been developed that reproduce this deactivation. The results for pure CaO follow nicely the model proposed by Wang et al.48 The model of Abanades et al. predicts the long-term stability of naturally occurring CaCO3 minerals, which deactivate even faster. 14 In contrast, for the dispersed CaO/Al2O3, sintering does not occur, and after 20 cycles about 90% of the adsorption efficiency is conserved. As shown above, the binding of CO2 to dispersed CaO/Al2O3 is weaker, and regeneration can be induced at much lower temperatures. Thus, the sorbent can be regenerated more efficiently at much lower temperatures by a mere CO2-partial pressure swing. Figure 2b shows the adsorption efficiency at 650 °C in alternating atmospheres of 50% CO2 in N2 and 100% N2, with 10 min allocated to each reaction for a total number of 85 cycles. The observed decrease in the sorbent’s activity is mostly due to the discrepancy in the kinetics of capture of CO2 versus that of its release. This is evident when a longer time for desorption is employed and the sorbent can be reactivated to its almost full efficiency by calcination at 850 °C in pure N2 atmosphere (Figure 2b, after 42 cycles). As mentioned above, a 30 min CO2-uptake period requires roughly 60 min for complete sorbent regeneration. The reversible decrease in sorbent activity, as seen in Figure 2b, can therefore be lowered if longer desorption times are allowed. Adsorption/desorption cycles can be done at lower temperatures, which is less energy intensive. However, the capture/ 4045

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Industrial & Engineering Chemistry Research desorption kinetics will differ, and longer regeneration periods are necessary. 3. CO2-Sorbent Binding Mechanism. The observation of CO2 desorbing from CaO/Al2O3 at much lower temperatures as compared to CO2 bound to bulk CaO points toward a different binding mechanism. To confirm this suggestion, diffuse reflectance infrared Fourier-transform spectroscopy has been used to gain insight into the CO2-sorbent interaction at a molecular level as there is a long-standing knowledge of CO2 on metal oxide surfaces.49,50 To discriminate between CO2 possibly bound to the γ-Al2O3 carrier or to CaO, a spectrum of CO2 on the CaO-free support is recorded at 300 °C. When the sample is exposed to the CO2 containing atmosphere, several weak signals become visible after some time, as shown in Figure 3. While the doublet between 2300 and 2400 cm-1 corresponds to the characteristic asymmetric stretch vibration ν3(OCO)a of gas-phase CO2, the bands between 1200 and 1700 cm-1 can be attributed to surface-bound species. The bands at 1649, 1440, and 1230 cm-1 are assigned to the asymmetric stretch ν2(OCO)a, the symmetric stretch ν3(OCO)s, and the δ(C-OH) modes of bridged bicarbonate, in excellent agreement with literature for CO2 adsorption on γ-Al2O3 at room temperature.51 The bicarbonate species results from reaction of CO2 with surface hydroxyl groups from the acidic alumina surface. These three bands disappear almost instantly upon

Figure 3. DRIFT spectrum of γ-Al2O3 after 30 min of exposure to a 15% CO2/N2 gas flow at 300 °C.

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switching to pure N2, indicating a very weak binding interaction. The other two bands at 1518 and 1300 cm-1 can be assigned to the asymmetric stretch vibrations ν3(OCO)a of weakly surfacebound carbonate species. The observed splitting of the ν3 mode, due to structural distortion from D3h symmetry,52 has been argued to reflect the coordination of the carbonate species to the surface.49,53 A value of Δν3 of 109 cm-1 would point toward a monodentate binding geometry. This species is more strongly bound as compared to the bicarbonate and persists for a longer time once pure nitrogen enters the cell. Figure 4 shows the DRIFT spectra for CO2 on bulk CaO powder (Figure 4a) and on the dispersed CaO/Al2O3 (Figure 4b). The samples have been dried at 450 °C for 60 min and exposed to a 15% CO2/N2 gas flow for 60 min. The spectra have been recorded 1 min after the flow had been switched to N2 only. The main vibrational features in the spectrum of CO2 on bulk CaO at 300 °C can be explained by the presence of a strongly bound carbonate species. The ν3(OCO)a is again split into two vibrational bands at 1373 and 1582 cm-1, which again indicates a distortion of the carbonate from its D3h symmetry upon binding to the calcium oxide surface. The band at 856 cm-1 corresponds to the IR-active out-of-plane ν2 mode, which is found at 879 cm-1 for the free carbonate ion.52 The band at 1071 cm-1 lies in the region of the symmetric stretch vibration ν1(OCO)s of free carbonate at 1063 cm-1. This breathing mode is Raman-active but IR-inactive in the D3h carbonate ion. The strong interaction with the sorbent might lead to a sufficient distortion of the carbonate so that this band obtains substantial IR intensity. This carbonate species is already so strongly bound that CO2 is not released from it at 300 °C. Much higher temperatures are required for sorbent regeneration as revealed by the TGA studies (see Figure 1). The experimental findings are in good agreement with recent experimental FT-IR studies at room temperature, in which bands around 1560, 1390, and 1069 cm-1 have been assigned to unidentate carbonate on CaO surfaces.54,55 In these studies, the band at 1069 cm-1 has been assigned to the C-O-stretch vibration between carbon and the oxygen atom, which is bound to Ca. Additional bands around 1630, 1460, and 1213 cm-1 have been assigned to bicarbonate species.54,55 In the 300 °C spectrum in Figure 4a, such bands seem to be entirely absent, excluding the presence of bicarbonates. A reason could be that at 300 °C the CaO surface is efficiently dehydroxylated, while in the room-temperature studies background water leads to the presence of a substantial amount of Ca(OH)2 on the surface.

Figure 4. DRIFT spectrum of CO2 on bulk CaO powder (panel a) and CaO/Al2O3 (panel b) at 300 and 450 °C. 4046

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Figure 5. CO2 uptake by 5.7 wt % CaO/Al2O3 at 300 and 450 °C as monitored by the intensity of the ν3 vibration of the formed carbonate.

An additional feature at 1776 cm-1, absent in the spectrum taken at 300 °C, has been tentatively assigned to a bridging carbonate species, in which the carbonates bind to two adjacent metal sites.54,56 It is known that the IR spectrum of CO2 on CaO depends strongly on temperature and CO2-partial pressure.57 Three different carbonate species have been identified in transmission IR studies at temperatures between room temperature and 460 °C. On the basis of these experimental findings, it has been argued that at room temperature a unidentate species is formed, which rearranges at higher temperature to yield two different bidentate carbonates.57 A recent density-functional theory study has put into question the assignment of bidentate carbonates on CaO and explained most experimental features with the presence of two different unidentate carbonates bound to CaO edge and corner sites.58 The temperature dependence of the CaO 3 CO2 system is also evident upon comparison of the IR spectra taken 300 and 450 °C, shown in Figure 4a. Increasing the temperature, the ν3 splitting decreases, the ν1 band decreases in intensity, while the ν2 band increases and seems to split. The spectrum seems to evolve toward the ones of the crystalline phases of CaCO3 like vaterite and calcite, in which the ν3 splitting is even smaller, ν1 is barely visible, and ν2 is a sharp intense band, split in the case of calcite.59 It has been noted before that the carbonate, which is formed upon reaction of CaO with CO2 at lower temperature, is amorphous and that the crystalline phases are formed at higher temperatures.60 The most notable difference, however, is the appearance of a sharp band at 1790 cm-1. This band has been argued to be due to bridged carbonates,54 which could be an intermediate structure between the initial unidentate carbonate and the crystalline phases. All these different spectral features reflect the complex potential-energy surface of this system on its way to crystalline carbonate formation. The situation changes drastically for CO2 on dispersed CaO/ Al2O3, as shown in Figure 4b. The only vibrational bands are found at 1630 and 1340 cm-1 and are assigned to the ν3 doublet of a carbonate species. The splitting Δν3 is 135 cm-1 and larger as compared to CO2 on bulk CaO. The ν1 and ν2 bands are completely absent or fall into noise, similar to the loosely bound carbonate on γ-Al2O3 (see above). Especially the absence of the breathing mode ν1, which becomes IR-active only upon the strong interaction with the sorbent, points to a much weaker binding interaction of CO2 with dispersed as compared to bulk

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Figure 6. DRIFT spectra of CO2 on CaO/Al2O3 at 300 °C with different CaO loadings.

CaO. Besides a small line broadening, the spectrum does not change at all with the temperature, again in marked contrast to the situation shown in Figure 4a. Obviously, there is only one energetically feasible, rather weak binding interaction possible between CO2 and CaO/Al2O3, and the formation of strongly bound carbonates or crystalline calcium carbonates, requiring energy and cost-intensive regeneration, is effectively inhibited. In fact, the facile decomposition of the formed, loosely bound carbonate, is accompanied with the desorption of CO2 and can be monitored by plotting the intensity of the band at 1630 cm-1 against time as shown in Figure 5. It can be seen that the IR signal for the measurement at 300 °C follows exactly the TG signal of the thermogravimetric analysis shown in Figure 1b. Obviously, the reaction conditions are very similar in the FT-IR cell and in the thermogravimetric analyzer. At 450 °C, the rate of desorption becomes faster, again in agreement with the TGA results. The intensity of the band at 1340 cm-1 shows time dependence identical to that of the band at 1630 cm-1. In the case that multiple adsorbates with different binding energies are formed, as indicated by the TGA measurements (see above), we are not able to distinguish between them spectroscopically. The absence of any vibrational bands characteristic for bridged or crystalline carbonates could be explained by the high dispersion of CaO on the 5.7 wt % CaO/Al2O3 support. Based on the nominal CaO loading, the BET surface area (Table 1), and the assumption that CaO remains on the surface of the support, the CaO density is ∼5.3 CaO/nm2. For the sorbent containing 20.3 wt % of CaO, a density of over 50 CaO/nm2 is obtained, a value clearly above monolayer coverage. In Figure 6, the IR spectra for the two different loadings are compared. The spectra are very similar, and neither the 20.3 wt % CaO/Al2O3 sorbent shows any clear bands that would point to strongly bound, bridged, or crystalline carbonates.

’ CONCLUSIONS Dispersed CaO/Al2O3 can be conveniently synthesized using incipient wetness impregnation of γ-Al2O3 and subsequent calcination at 700 °C. The resulting material is an effective CO2-sorbent at intermediate temperatures between 300 and 650 °C, which is characterized by fast uptake kinetics. In contrast to bulk CaO, the dispersed CaO/Al2O3 can be regenerated under rather mild conditions, by temperature swing or by a CO2-partial 4047

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Industrial & Engineering Chemistry Research pressure swing. This leads to an excellent long-term stability as shown in cycling experiments up to 84 cycles and makes this sorbent a promising candidate for the sorption-enhanced production of hydrogen via steam reforming and water-gas shift reactions. The binding interaction between CO2 and dispersed CaO/ Al2O3 has been studied in detail by infrared spectroscopy. At 300 °C, only rather loosely bound carbonates and bicarbonates are formed on the γ-Al2O3 carrier. On bulk CaO, the IR spectra show strongly bound unidentate carbonates at 300 °C, which partially rearrange at 450 °C to form bridged carbonates and evolve toward the crystalline phases. The dispersed CaO/Al2O3 adsorbs CO2 exclusively as weakly and reversibly bound carbonates independent of temperature and loading and inhibits formation of strongly bound carbonates. While the CO2-sorbent binding interaction could be probed conveniently by IR spectroscopy, at this point the molecular structure of the dispersed CaO/Al2O3 itself remains unclear. The drastically different binding of CO2 to bulk CaO powder and dispersed CaO/Al2O3 could be a size effect of the CaO islands. A second possibility could be the formation of mixed oxide phases like Ca3Al2O6 from CaO and γ-Al2O3 during the calcination at 700 °C, although the formation of Ca3Al2O6 usually requires temperatures above 1000 °C.61,62 Other authors have shown that Al2O3 will react with CaO and form bulk Ca12Al14O33 already at 800 °C.19 In this work, although 700 °C is used for the calcination, the mesoporous γ-Al2O3 might be very active due to its high surface area, and thus reactions between CaO and Al2O3 could occur at lower temperatures. Preliminary results using XRD have not shown any signal for crystalline CaO or mixed CaAl oxides. This indicates either they do not exist or their domain size is below the current detection limit. Further XRD and TEM studies that could answer this question are in progress.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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